The document EP1965075A1 discloses a system comprising a doubly-fed inductor generator, a grid-side-converter, a grid-side-converter GSC, a machine-side-converter MSC, a DC link between both converters, a crowbar connected to rotor of the generator, a first control unit for controlling the crowbar depending on the DC link voltage, and a second control unit for controlling both converters GSC and MSC. The first and second controllers are independent of each other.
The document U.S. Pat. No. 8,373,293B2 discloses a system comprising a doubly-fed inductor generator, a grid-side-converter, a grid-side-converter GSC, a machine-side-converter MSC, a DC link between both converters, a chopper connected to the DC link, a crowbar connected to rotor of the generator, a converter controller for controlling both converters GSC and MSC, and a separate protection device for controlling both the chopper and the crowbar.